Optical monitoring in a two-step chemical mechanical polishing process

ABSTRACT

An optical monitoring system for a two-step polishing process which generates a reflectance trace for each of plurality of radial zones. The CMP apparatus may switch from a high-selectivity slurry to a low-selectivity slurry when any of the reflectance traces indicate initial clearance of the metal layer, and polishing may halt when all of the reflectance traces indicate that oxide layer has been completely exposed.

This application is a continuation (and claims the benefit of priorityunder 35 USC 120) of U.S. application Ser. No. 09/764,733, filed on Jan.16, 2001, now U.S. Pat. No. 6,506,097, which claims priority to U.S.Provisional Application Serial No. 60/176,645, filed on Jan. 18, 2000,the entire disclosures of which are incorporated by reference.

BACKGROUND

The present invention relates generally to chemical mechanical polishingof substrates, and more particularly to methods and apparatus fordetecting an end-point of a metal layer during a chemical mechanicalpolishing operation.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. After each layer is deposited, the layer is etchedto create circuitry features. As a series of layers are sequentiallydeposited and etched, the outer or uppermost surface of the substrate,i.e., the exposed surface of the substrate, becomes increasinglynon-planar. This non-planar surface presents problems in thephotolithographic steps of the integrated circuit fabrication process.Therefore, there is a need to periodically planarize the substratesurface.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing pad. Thepolishing pad may be either a “standard” pad or a fixed-abrasive pad. Astandard pad has a durable roughened surface, whereas a fixed-abrasivepad has abrasive particles held in a containment media. The carrier headprovides a controllable load, i.e., pressure, on the substrate to pushit against the polishing pad. A polishing slurry, including at least onechemically-reactive agent, and abrasive particles if a standard pad isused, is supplied to the surface of the polishing pad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness. Variations in the initial thickness ofthe substrate layer, the slurry composition, the polishing padcondition, the relative speed between the polishing pad and thesubstrate, and the load on the substrate can cause variations in thematerial removal rate. These variations cause variations in the timeneeded to reach the polishing endpoint. Therefore, the polishingendpoint cannot be determined merely as a function of polishing time.

One way to determine the polishing endpoint is to remove the substratefrom the polishing surface and examine it. For example, the substratemay be transferred to a metrology station where the thickness of asubstrate layer is measured, e.g., with a profilometer or a resistivitymeasurement. If the desired specifications are not met, the substrate isreloaded into the CMP apparatus for further processing. This is a timeconsuming procedure that reduces the throughput of the CMP apparatus.Alternatively, the examination might reveal that an excessive amount ofmaterial has been removed, rendering the substrate unusable.

Several methods have been developed for in-situ polishing endpointdetection. Most of these methods involve monitoring a parameterassociated with the substrate surface, and indicating an endpoint whenthe parameter abruptly changes. For example, where an insulative ordielectric layer is being polished to expose an underlying metal layer,the coefficient of friction and the reflectivity of the substrate willchange abruptly when the metal layer is exposed.

Where the monitored parameter changes abruptly at the polishingendpoint, such endpoint detection methods are acceptable. However, asthe substrate is being polished, the polishing pad condition and theslurry composition at the pad-substrate interface may change. Suchchanges may mask the exposure of an underlying layer, or they mayimitate an endpoint condition. Additionally, such endpoint detectionmethods will not work if only planarization is being performed, if theunderlying layer is to be over-polished, or if the underlying layer andthe overlying layer have similar physical properties.

SUMMARY

In one aspect, the invention is directed to a method of polishing asubstrate. A first layer of a substrate is chemical mechanical polishedwith a first polishing fluid. The substrate has a second layer disposedunder the first layer, and the first and second layers have differingreflectivity. The substrate is optically monitored during polishing withthe first polishing slurry to generate plurality of intensity traces.Each intensity trace includes intensity measurements from a differentradial range on the substrate. Once any of the intensity tracesindicates an initial clearance of the first layer, the substrate ischemical mechanical polished with a second polishing fluid havingdifferent polishing properties than the first polishing fluid. Opticalmonitoring of the substrate continues during polishing with the secondpolishing slurry, and polishing is halted after all the intensity tracesindicate that the second layer has been completely exposed.

Implementations of the invention may include one or more of thefollowing features. Optical monitoring may include directing a lightbeam through a window in a polishing surface and causing the light beamto move in a path across the substrate, monitoring a reflectance signalproduced by the light beam reflecting off the substrate, and extractinga plurality of intensity measurements from the reflectance signal.Generating the plurality of intensity traces may include sorting eachintensity measurement into one of the radial ranges according to aposition of the light beam during the intensity measurement anddetermining the intensity trace from the intensity measurementsassociated with the radial range. The first slurry may be ahigh-selectivity slurry and the second slurry may be a low-selectivityslurry. The first layer may be more reflective than the second layer.The first layer may be a metal layer, such as copper. The second layermay be an oxide layer, such as silicon dioxide, or a barrier layer, suchas tantalum or tantalum nitride.

In another aspect, the invention is directed to a method of polishing asubstrate in which a surface of a substrate is brought into contact witha polishing surface that has a window. The substrate has a first layerdisposed over a second layer, and the first and second layers havediffering reflectivity. A first slurry is supplied to the substrate fora first polishing step, and relative motion is caused between thesubstrate and the polishing surface. A light beam is directed throughthe window, and the motion of the polishing surface relative to thesubstrate causes the light beam to move in a path across the substrate.A reflectance signal produced by the light beam reflecting off thesubstrate is monitored, a plurality of intensity measurements areextracted from the reflectance signal, and a plurality of intensitytraces are generated with each intensity trace including intensitymeasurements from a different radial range on the substrate. A secondslurry is supplied to the substrate for a polishing second polishingstep when any of the intensity traces indicates an initial clearance ofthe first layer. The second slurry has different polishing propertiesthan the first slurry. Polishing is halted after all the intensitytraces indicate that the second layer has been completely exposed.

Implementations of the invention may include one or more of thefollowing features. The first slurry may be a high-selectivity slurryand the second slurry may be a low-selectivity slurry. The second layermay be disposed over a third layer in the substrate.

In another aspect, the invention is directed to a method of polishing asubstrate having a metal layer disposed over an oxide layer. In themethod, a surface of a substrate is brought into contact with apolishing surface that has a window. A high-selectivity slurry issupplied to the polishing surface. Relative motion is caused between thesubstrate and the polishing surface. A light beam is directed throughthe window, and the motion of the polishing surface relative to thesubstrate causes the light beam to move in a path across the substrate.A reflectance signal produced by the light beam reflecting off thesubstrate is monitored, and a plurality of intensity measurements areextracted from the reflectance signal. A radial position is determinedfor each intensity measurement, and the plurality of intensitymeasurements are divided into a plurality of radial ranges according tothe radial positions. A plurality of intensity traces are generated,with each intensity trace including intensity measurements from one ofthe plurality of radial ranges. A low-selectivity slurry is supplied tothe polishing surface when any of the intensity traces indicates aninitial clearance of the metal layer, and polishing is halted when allthe intensity traces indicate that the oxide layer has been completelyexposed.

Implementations of the invention may include one or more of thefollowing features. A sudden drop in a reflectance trace may indicatesinitial clearance of the metal layer in the radial range associated withthe reflectance trace. A flattening out of a reflectance trace mayindicate exposure of the oxide layer in the radial range associated withthe reflectance trace. The substrate may includes a barrier layer, e.g.,tantalum or tantalum nitride, disposed between the metal layer, e.g.,copper, and the oxide layer, e.g., silicon oxide.

Advantages of the invention include one or more of the following. Acopper-coated substrate may be polished with reduced dishing anderosion. Both the polishing endpoint, and the point at which thepolishing apparatus should switch from a high-selectivity slurry to alow-selectivity slurry may be accurately determined. Detailed data isavailable on the progress of the metal polishing operation at differentlocations of the wafer.

Other features and advantages of the invention will become apparent fromthe following description, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a chemical mechanicalpolishing apparatus.

FIG. 2 is a side view of a chemical mechanical polishing apparatusincluding-an optical reflectometer.

FIG. 3 is a simplified cross-sectional view of a substrate beingprocessed, schematically showing a laser beam impinging on andreflecting from the substrate.

FIG. 4 is a graph showing a measured reflectance trace in arbitraryintensity units (a.u.).

FIGS. 5A-5E are simplified plan views illustrating the position of awindow in a polishing pad as a platen rotates.

FIG. 6 is a flow chart of a method of determining the end-point of thepolishing of a metal layer during CMP.

FIG. 7A is a schematic view illustrating the path of a laser beneath thecarrier head.

FIG. 7B is a graph showing a hypothetical portion of a reflectance tracegenerated by a single sweep of the window beneath the carrier head.

FIG. 8 is a schematic view illustrating the radial positions of samplingzones from the path of the laser.

FIG. 9A is a flow chart of a method of determining the radial positionof a sampling zone.

FIG. 9B is a graph showing the time at which the laser beam passesbeneath the leading and trailing edges of the substrate as a function ofthe number of rotations of the platen.

FIG. 10 is a schematic view illustrating the calculation of the radialposition of the sampling zones.

FIG. 11 is a schematic diagram of a data structure to store intensitymeasurements.

FIG. 12 is a graph illustrating an overlay of several reflectance tracestaken at different times.

FIGS. 13A-13H are graphs showing the reflected intensity of the metallayer as a function of distance from the center of the substrate over apolishing period.

FIG. 14 is a simplified cross-sectional view of a substrate with abarrier layer.

FIGS. 15A and 15B are graphs illustrating reflectance traces duringpolishing of the substrate shown in FIG. 14.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, one or more substrates 10 may be polished bya CMP apparatus 20. A description of a similar polishing apparatus 20may be found in U.S. Pat. No. 5,738,574, the entire disclosure of whichis incorporated herein by reference. Polishing apparatus 20 includes aseries of polishing stations 22 and a transfer station 23. Transferstation 23 serves multiple functions, including receiving individualsubstrates 10 from a loading apparatus (not shown), washing thesubstrates, loading the substrates into carrier heads, receiving thesubstrates from the carrier heads, washing the substrates again, andfinally, transferring the substrates back to the loading apparatus.

Each polishing station includes a rotatable platen 24 on which is placeda polishing pad 30. The first and second stations may include atwo-layer polishing pad with a hard durable outer surface, whereas thefinal polishing station may include a relatively soft pad. If substrate10 is an “eight-inch” (200 millimeter) or “twelve-inch” (300 millimeter)diameter disk, then the platens and polishing pads will be about twentyinches or thirty inches in diameter, respectively. Each platen 24 may beconnected to a platen drive motor (not shown). For most polishingprocesses, the platen drive motor rotates platen 24 at about thirty totwo hundred revolutions per minute, although lower or higher rotationalspeeds may be used. Each polishing station may also include a padconditioner apparatus 28 to maintain the condition of the polishing padso that it will effectively polish substrates.

Polishing pad 30 typically has a backing layer 32 which abuts thesurface of platen 24 and a covering layer 34 which is used to polishsubstrate 10. Covering layer 34 is typically harder than backing layer32. However, some pads have only a covering layer and no backing layer.Covering layer 34 may be composed of an open cell foamed polyurethane ora sheet of polyurethane with a grooved surface. Backing layer 32 may becomposed of compressed felt fibers leached with urethane. A two-layerpolishing pad, with the covering layer composed of IC-1000 and thebacking layer composed of SUBA-4, is available from Rodel, Inc., ofNewark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.).

A rotatable multi-head carousel 60 is supported by a center post 62 andis rotated thereon about a carousel axis 64 by a carousel motor assembly(not shown). Center post 62 supports a carousel support plate 66 and acover 68. Carousel 60 includes four carrier head systems 70. Center post62 allows the carousel motor to rotate carousel support plate 66 and toorbit the carrier head systems and the substrates attached thereto aboutcarousel axis 64. Three of the carrier head systems receive and holdsubstrates, and polish them by pressing them against the polishing pads.Meanwhile, one of the carrier head systems receives a substrate from anddelivers a substrate to transfer station 23.

Each carrier head system includes a carrier or carrier head 80. Acarrier drive shaft 74 connects a carrier head rotation motor 76 (shownby the removal of one quarter of cover 68) to each carrier head 80 sothat each carrier head can independently rotate about it own axis. Thereis one carrier drive shaft and motor for each head. In addition, eachcarrier head 80 independently laterally oscillates in a radial slot 72formed in carousel support plate 66. A slider (not shown) supports eachdrive shaft in its associated radial slot. A radial drive motor (notshown) may move the slider to laterally oscillate the carrier head.

The carrier head 80 performs several mechanical functions. Generally,the carrier head holds the substrate against the polishing pad, evenlydistributes a downward pressure across the back surface of thesubstrate, transfers torque from the drive shaft to the substrate, andensures that the substrate does not slip out from beneath the carrierhead during polishing operations.

Carrier head 80 may include a flexible membrane 82 that provides amounting surface for substrate 10, and a retaining ring 84 to retain thesubstrate beneath the mounting surface. Pressurization of a chamber 86defined by flexible membrane 82 forces the substrate against thepolishing pad. Retaining ring 84 may be formed of a highly reflectivematerial, or it may be coated with a reflective layer to provide it witha reflective lower surface 88. A description of a similar carrier head80 may be found in U.S. patent application Ser. No. 08/745,679, entitleda CARRIER HEAD WITH a FLEXIBLE MEMBRANE FOR a CHEMICAL MECHANICALPOLISHING SYSTEM, filed Nov. 8, 1996, by Steven M. Zuniga et al.,assigned to the assignee of the present invention, the entire disclosureof which is incorporated herein by reference.

A slurry 38 containing a reactive agent (e.g., deionized water for oxidepolishing) and a chemically-reactive catalyzer (e.g., potassiumhydroxide for oxide polishing) may be supplied to the surface ofpolishing pad 30 by a slurry supply port or combined slurry/rinse arm39. If polishing pad 30 is a standard pad, slurry 38 may also includeabrasive particles (e.g., silicon dioxide for oxide polishing).

In operation, the platen is rotated about its central axis 25, and thecarrier head is rotated about its central axis 81 and translatedlaterally across the surface of the polishing pad.

A hole 26 is formed in platen 24 and a transparent window 36 is formedin a portion of polishing pad 30 overlying the hole. Transparent window36 may be constructed as described in U.S. patent application Ser. No.08/689,930, entitled METHOD OF FORMING A TRANSPARENT WINDOW IN APOLISHING PAD FOR A CHEMICAL

MECHANICAL POLISHING APPARATUS by Manoocher Birang, et al., filed Aug.26, 1996, and assigned to the assignee of the present invention, theentire disclosure of which is incorporated herein by reference. Hole 26and transparent window 36 are positioned such that they have a view ofsubstrate 10 during a portion of the platen's rotation, regardless ofthe translational position of the carrier head.

A reflectometer 40 is secured to platen 24 generally beneath hole 26 androtates with the platen. The reflectometer includes a light source 44and a detector 46. The light source generates a light beam 42 whichpropagates through transparent window 36 and slurry 38 (see FIG. 3) toimpinge upon the exposed surface of substrate 10. For example, the lightsource 44 may be laser and the light beam 42 may be a collimated laserbeam. The light laser beam 42 is projected from laser 44 at an angle afrom an axis normal to the surface of substrate 10, i.e., at an angle afrom axes 25 and 81. In addition, if the hole 26 and window 36 areelongated, a beam expander (not illustrated) may be positioned in thepath of the light beam to expand the light beam along the elongated axisof the window. Laser 44 may operate continuously. Alternatively, thelaser may be activated to generate laser beam 42 during a time when hole26 is generally adjacent substrate 10.

Referring to FIGS. 2 and 5A-5E, CMP apparatus 20 may include a positionsensor 160, such as an optical interrupter, to sense when window 36 isnear the substrate. For example, the optical interrupter could bemounted at a fixed point opposite carrier head 80. A flag 162 isattached to the periphery of the platen. The point of attachment andlength of flag 162 is selected so that it interrupts the optical signalof sensor 160 from a time shortly before window 36 sweeps beneathcarrier head 80 to a time shortly thereafter. The output signal fromdetector 46 may be measured and stored while the optical signal ofsensor 160 is interrupted.

In operation, CMP apparatus 20 uses reflectometer 40 to determine theamount of material removed from the surface of the substrate, or todetermine when the surface has become planarized. A general purposeprogrammable digital computer 48 may be connected to laser 44, detector46 and sensor 160. Computer 48 may be programmed to activate the laserwhen the substrate generally overlies the window, to store intensitymeasurements from the detector, to display the intensity measurements onan output device 49, to store the intensity measurement, to sort theintensity measurements into radial ranges, and to detect the polishingendpoint.

Referring to FIG. 3, a substrate 10 includes a silicon wafer 12 and anoverlying metal layer 16 disposed over an oxide or nitride layer 14. Themetal may be copper, tungsten, aluminum, among others. As differentportions of the substrate with different reflectivities are polished,the signal output from the detector 46 varies with time. Particularly,when the metal layer 16 has been polished away to expose the oxide ornitride layer 14, the reflectivity of the substrate drops. The timevarying output of detector 46 may be referred to as an in-situreflectance measurement trace (or more simply, a reflectance trace). Asdiscussed below, this reflectance trace may be used to determine theend-point of the metal layer polishing operation.

Referring to FIGS. 4 and 5A-5E, a measured reflectance trace with atransient intensity waveform 90 generated by polishing a metal-coatedwafer is shown. The intensity waveform 90 is generated over a relativelylong time scale (measured in seconds). Characteristic features of thewaveform include top level plateau 97, each of which is surrounded byleft and right intermediate plateau 98. One cycle of the waveform 90includes left and right intermediate level plateau 98, one of the toplevel plateau 97, and a background level 94.

The intermediate plateau 98 represent reflections from the retainingring 84, while the top level plateau 97 represent reflections from thesubstrate 10. The background level represents scattered reflections fromthe window and slurry. The reflection from retaining ring 84 is higherthan background level. As the substrate 10 is polished and the metallayer 16 is removed to expose the underlying layer 14, the end-pointwaveform 90 drops toward or below the level of the intermediate plateau98.

Referring to FIGS. 4 and 5A-5E, the large scale structure of reflectancetrace 90 can be explained by reference to the angular position of platen24. Initially, window 36 does not have view of the substrate (see FIG.5A). Consequently, laser beam 42 is not reflected and the intensitymeasured by detector 46 is a result of background intensity, includingreflection from slurry 38 and transparent window 36. This low intensitycorresponds to the background level 94. As platen 24 rotates, window 36first sweeps underneath retaining ring 84 of carrier head 80 (see FIG.5B). The lower surface 88 of retaining ring 84 reflects a portion oflaser beam 42 into detector 46, creating an intermediate intensitymeasurement that corresponds to intermediate plateau 98. As window 36sweeps beneath substrate 10 (see FIG. 5C) a portion of laser beam 42 isreflected by the substrate. In general, the metal layer of substrate 10will have a high reflectivity, resulting in top level plateau 97 onreflectance trace 90. As the platen continues to rotate, window 36passes again beneath retaining ring 84 (see FIG. 5D). Finally, window 36sweeps out from beneath carrier head 80 (see FIG. 5E), and the detectormeasures a low intensity that corresponds to the background 94.

Computer 48 of CMP apparatus 20 may use the reflectance trace generatedby reflectometer 40 to determine the end-point of the metal layerpolishing operation. Each measurement may be performed at a plurality ofradial positions. In addition, computer 48 may use the intensitymeasurements to determine the flatness of the substrate and thepolishing uniformity for CMP tool and process qualification as explainedbelow.

Referring now to FIG. 6, an end-point determining process is shown.First, several polishing parameters that will be used during theend-point determination are stored in the memory of computer 48 (step101). The polishing parameters of interest include the platen rotationrate and the carrier head sweep profile.

A metal layer on a surface of the substrate 12 is polished (step 102) bybringing the surface of the substrate into contact with the polishingpad 30 (FIG. 2). The polishing pad 30 is rotated, causing relativemotion between the substrate and the polishing pad.

Transient intensity data is monitored and collected for a plurality ofsampling zones (step 104). This is done by directing a light beamgenerated by the reflectometer 40 through the window. The motion of thepolishing pad 30 relative to the substrate 12 causes the light beam tomove in a path across the substrate surface. Light beam reflections fromthe substrate 10 and the retaining ring 84 are detected by a sensor,which generates reflection data associated with the light beamreflections.

The transient intensity data is displayed on a monitor (step 106) for anoperator to monitor the progress of the polishing operation. A patternrecognizer is applied to the transient intensity data to detect signalchanges (step 108). The pattern recognizer may simply be a thresholddetector which checks whether the intensity data has fallen below apredetermined threshold. Alternatively, in another embodiment, a windowlogic can be applied to the data to detect a sequence of signal changes.Three types of window logic are used to detect local maxima and minima:a window logic with a downwardly cusp to detect a downward trend in thereflection data; a window logic with an upwardly cusp to detect anupward trend in the reflection data; and a window logic with asubstantially flat line to detect that the reflection data is relativelystatic. The signal changes may be averaged. More discussion of patternrecognition algorithms for endpoint detection may be found in abovementioned U.S. patent application Ser. No. 08/689,930.

The output of the pattern recognizer is a stop signal which, along withadditional feedback data, is provided to a polisher controller (step110). The polisher controller uses the feedback data to adjust variousvariables and parameters to minimize erosion and dishing of the surfacelayer. For instance, the polishing pressure, polishing speed, chemistry,and slurry composition may be deployed to optimize the overall polishingperformance and/or polishing quality. The stop signal causes thepolisher controller to stop the current metal layer polishing operation(step 112).

Concurrent with steps 106-112, the process of FIG. 6 stores thetransient intensity data onto a data storage device, e.g., a computerdisk (step 114) for subsequent processing. In brief, the intensity foreach sampling zone is determined (step 116), the radial position of eachsampling zone is calculated (step 118), and the intensity measurementsare sorted into radial ranges (step 150). The sorted intensitymeasurements are used to measure the polishing uniformity and removalrates at different radial ranges of the substrate (step 152). Each ofthese steps will be discussed in greater detail below.

Generally, the reflected intensity changes during polishing fordifferent radial positions on the substrate. The metal layer may beremoved at different rates for different portions of the substrate. Forinstance, the metal layer near the center of the substrate may beremoved last, while the metal layer near the perimeter or edge of thesubstrate may be removed first, or vice versa. The reflection data fromthe entire wafer is captured at a relatively fine time scale in theorder of milliseconds and is available for experimentation to improvethe deposition process. By analyzing the recorded data, the process canbe changed to make it faster, shorter or smoother. As can beappreciated, the stored data is useful for process research anddevelopment to optimize the process performance.

Referring to FIGS. 7A and 7B, the combined rotation of the platen andthe linear sweep of the carrier head causes window 36 (and thus laserbeam 42) to sweep across the bottom surface of carrier head 80 andsubstrate 10 in a sweep path 120. As the laser beam sweeps across thesubstrate, reflectometer 40 integrates the measured intensity over asampling period, Tsample, to generate a series of individual intensitymeasurements Ia, Ib, . . . Ij. The sample rate F (the rate at whichintensity measurements are generated) of reflectometer 40 is given byF=1/Tsample. Reflectometer 40 may have a sample rate between about 10and 400 Hertz (Hz), corresponding to a sampling period between about 2.5and 100 milliseconds. Specifically, reflectometer 40 may have a samplingrate of about 40 Hz and a sampling period of about 25 milliseconds.

Thus, each time that laser 44 is activated, reflectometer 40 measuresthe intensity from a plurality of sampling zones 122 a-122 j. Eachsampling zone corresponds to the area of the substrate over which thelaser beam sweeps during a corresponding sampling period. In summary, instep 106, reflectometer 40 generates a series of intensity measurementsIa, Ib, . . . Ij corresponding to sampling zones 122 a, 122 b, . . . ,122 j.

Although FIG. 7A illustrates ten sampling zones, there could be more orfewer zones, depending on the platen rotation rate and the samplingrate. Specifically, a lower sampling rate will result in fewer, widersampling zones, whereas a higher sampling rate will result in a greaternumber of narrower sampling zones. Similarly, a lower rotation rate willresult in a larger number of narrower sampling zones, whereas a higherrotation rate will result in a lower number of wider sampling zones. Inaddition, multiple detectors could be used to provide more samplingzones.

As shown in FIG. 7B, the intensity measurements Ia and Ij for samplingzones 122 a and 122 j, respectively, are low because window 36 does nothave a view of the carrier head, and consequently laser beam 42 is notreflected. Sampling zones 122 b and 122 i are located beneath retainingring 84, and therefore intensity measurements Ib and Ii will be ofintermediate intensity. Sampling zones 122 c, 122 d, . . . 122 h arelocated beneath the substrate, and consequently generate relativelylarge intensity measurements Ic, Id, . . . Ih at a variety of differentradial positions across the substrate.

FIG. 12 is an overlay of several transient signal graphs 300-320. Eachof the transient signal graphs 300-320 represents intensity data over aninterval associated with a sweep of the window beneath the carrier head.For instance, the graph 300 shows the end-point data between about 1.7seconds to about 2.7 seconds, and the graph 320 shows the end-point databetween about 350.8 seconds and about 351.8 seconds. Of course, thetransient signal graphs can be stored in computer 48 for laterreference.

FIG. 12 shows how the endpoint reflected intensity signal changes duringthe polishing operation. Initially, in period 300, the metal layer onthe surface of the substrate 10 is jagged. The metal layer 16 has someinitial topography because of the topology of the underlying patternedlayer 14. Due to this topography, the light beam scatters when itimpinges the metal layer. As the polishing operation progresses, themetal layer becomes more planar and the reflectivity of the polishedmetal layer increases during periods 302-308. As such, the signalstrength steadily increases to a stable level. From period 310-320, asthe metal layer 16 is increasingly cleared to expose the oxide layer 14,the overall signal strength declines until the polishing operation iscompleted. Thus, in period 320, only a small trace of metal remains inthe center of the substrate 10.

When entire surface of the substrate is covered with a metal layer, suchas copper, the reflection from the substrate 10 has a square profile. Asthe metal layer is removed from the edge of the substrate 10, theprofile of the reflection from the substrate takes on a trapezoidalshape. Eventually, when the metal layer is nearly removed by thepolishing operation, the profile of the reflection from the substrate 10takes on a triangular shape.

The transient signal graphs 300-320 can be viewed by the operator on thedisplay 49 either during or after the polishing operation. The operatorcan use the displayed transient signal graphs for a variety ofdiagnostic and process control decisions (which may be applicable toboth reflectivity measurements in metal polishing and interferencemeasurements in oxide polishing). The transient signal graphs can beused to select process parameters in order to optimize polishinguniformity. For example, a test wafer can be polished when initiallyselecting process parameters, such as the plate rotation rate, carrierhead pressure, carrier head rotation rate, carrier head sweep profile,and slurry composition. High reflectivity areas represent regions wheremetal remains on the substrate, and low reflectivity area representregions where metal has been removed from the substrate. A noisytransient signal graph indicates that the metal has not been evenlyremoved from the substrate, whereas a relatively flat transient signalgraph indicates uniform polishing. Consequently, the operator can drawimmediate conclusions, without resorting to measuring the substratelayer thickness with a metrology tool, regarding the effectiveness ofthe selected process parameters. The operator can then adjust thepolishing parameters, polish another test wafer, and determine whetherthe new polishing parameters have improved the polishing uniformity.

An operator may also examine the transient signal graphs to determinewhether the substrate has been polished to planarity, and whetherpolishing should be halted. Furthermore, if an operator notes duringpolishing of an actual device wafer that a portion of the substrate isbeing polished too slowly or too quickly, the process parameters can bechanged while polishing is in progress to adjust the polishing rateprofile.

The transient signal graphs can also be used as a measure of processrepeatability. For example, if the transient signal graphs departsignificantly from their expected shapes, this indicates that there issome problem in the polishing machine or process.

In addition, the transient signal graphs can be used to “qualify” aprocess. Specifically, when the polishing machine receives a new set ofconsumables, e.g., if the polishing pad or slurry is replaced, theoperator may wish to verify that the polishing uniformity has not beenaffected. An operator can compare the transient signal graphs for thesubstrates polished before and after the change in consumables todetermine whether the polishing uniformity has been affected.

Turning now to FIG. 8, in step 108 the radial positions Ra, Rb, . . . Rjof the corresponding sampling zones 122 a, 122 b, . . . 122 j aredetermined. One way to determine the radial position of a sampling zoneis to calculate the position of the laser beneath the substrate based onthe measurement time Tmeasure and the platen rotation rate and carrierhead sweep profile. Unfortunately, the actual platen rotation rate andcarrier head sweep profile may not precisely match the polishingparameters. Therefore, a preferred method 130 of determining the radialpositions of the sampling zones is shown in FIG. 9A. First, the timeTsym at which laser beam 42 passes beneath a mid-line 124 (see FIG. 5C)of the substrate is determined (step 132). Then the radial positions ofthe sampling zones are determined from the time difference between themeasurement time Tmeasure and the symmetric time Tsym (step 134).

One method of determining the symmetry time Tsym is to average the timesof the first and last large intensity measurements from each sweep, asthese intensity measurements should correspond to the substrate edge.However, this results in some uncertainty in Tsym because the positionof the sampling zones beneath the substrate are not known.

Referring to FIG. 9B, in order to compute the symmetric time Tsym instep 132, computer 48 determines the first and last large intensitymeasurements from sweep path 120, i.e., intensity measurements Ic andIh, and stores the corresponding measurement times Tlead and Ttrail.These lead and trail times Tlead and Ttrail are accumulated on eachsweep to generate a series of lead times Tlead1, Tlead2, . . . TleadNand trail times Ttrail1, Ttrail2, . . . TtrailN. Computer 48 stores leadtimes Tlead1, Tlead2, . . . TleadN and the associate number of platenrotations 1, 2, . . . N for each leading spike 96. Similarly, computer48 stores the trail times Ttrail1, Ttrail2, . . . TtrailN and theassociated number of rotations 1, 2, . . . N of each trailing spike 98.Assuming that platen 24 rotates at a substantially constant rate, thetimes Tlead 1, Tlead 2, . . . TleadN form a substantially linearincreasing function (shown by line 136). Similarly, the times Ttrail1,Ttrail2, . . . TtrailN also form a substantially linear increasingfunction (shown by line 137). Computer 48 performs two least square fitsto generate two linear functions Tlead(n) and Ttrail(n) as follows:

Tlead(n)=a1+(a2*n)

Ttrail(n)=a3+(a4*n)

where n is the number of platen rotations and a1, a2, a3 and a4 arefitting coefficients calculated during the least square fit. Once thefitting coefficients have been calculated, the symmetry time Tsym atwhich laser beam 42 crosses mid-line 124 (shown by phantom line 138) maybe calculated as follows:$T_{sym} = {\frac{a_{1} + a_{3}}{2} + {\left( \frac{a_{2} + a_{4}}{2} \right)\quad n}}$

By using a least square fit over several platen rotations to calculatethe symmetry time Tsym, uncertainty caused by the differences in therelative position of the sampling zone beneath the retaining ring aresubstantially reduced, thereby significantly reducing uncertainty in thesymmetry time Tsym.

Once computer 48 has calculated the time Tsym at which laser beam 42crosses midline 124, the radial distance Ra, Rb, . . . Rj of eachsampling zone 122 a, 122 b, . . . 122 j from the center 126 of thesubstrate are calculated in step 132. Referring to FIG. 10, the radialposition may be calculated as follows:

R={square root over (d²+L²+2dLcosθ)}

where d is the distance between the center of the polishing pad and thecenter of window 36, L is the distance from the center of the polishingpad to the center of substrate 10, and θ is the angular position of thewindow. The angular position θ of the window may be calculated asfollows:

 θ=f_(platen)·2π(T _(measure) −T _(sym))

where f_(platen) is the rotational rate of the platen (in rpm). Assumingthat the carrier head moves in a sinusoidal pattern, the linear positionL of the carrier head may be calculated as follows:

L=L ₀ +A·cos(ω·T _(measure))

where ω is the sweep frequency, A is the amplitude of the sweep, and LOis the center position of the carrier sweep.

In another embodiment, position sensor 160 could be used to calculatethe time Tsym when the window crosses midline 124. Assuming that sensor160 is positioned opposite carrier head 80, flag 162 would be positionedsymmetrically across from transparent window 36. The computer 48 storesboth the trigger time Tstart when the flag interrupts optical beam ofthe sensor, and the trigger time Tend when the flag clears the opticalbeam. The time Tsym may be calculated as the average of Tstart and Tend.In yet another embodiment, the platen and carrier head positions couldbe determined at each sample time Ta, Tb, . . . Th, from opticalencoders connected to the platen drive motor and radial drive motor,respectively.

Once the radial positions Ra, Rb, . . . Rm of the sampling zones havebeen calculated, some of the intensity measurement may be disregarded.If the radial position R of a sampling zone is greater than the radiusof the substrate, then the intensity measurement for that sampling zoneincludes mostly radiation reflected by the retaining ring or backgroundreflection from the window or slurry. Therefore, the intensitymeasurements for any sampling zone that is mostly beneath the retainingring is ignored. This ensures that spurious intensity measurements arenot used in the calculation of the thin film layer reflected intensity.

After several sweeps of laser beam 42 beneath the substrate, computer 48accumulates a set of intensity measurements I1, 12, . . . IN, eachassociated with a measurement time T1, T2, . . . TN, and a radialposition R1, R2, . . . RN. Referring to FIG. 11, as the intensity, time,and radial position measurements are accumulated in steps 106 and 108,the time and intensity measurements are sorted into bins in a datastructure 140 in step 110. Each bin is associated with a radial range ofsampling zones. For example, intensity measurements for sampling zoneslocated up to 20 mm from the center of the substrate may be placed in afirst bin 142 (see FIG. 13A) which is discussed below, intensitymeasurements made for sampling zones located between 20 and 30 mm fromthe center of the substrate may be placed in a second bin 144 (see FIG.13B), intensity measurements made for sampling zones located between 30and 40 mm from the center of the substrate may be placed in a third bin146 (see FIG. 13C), and so on. The exact number of bins and the radialranges of the bins depend upon the information that the user desires toextract. In general, the radial range of each bin may be selected sothat a sufficient number of intensity measurements are accumulated inthe bin to provide visually meaningful information.

The calculations discussed above are performed for each bin, therebyproviding reflected intensity measurements at a plurality of radialpositions across the surface of the substrate. Graphs of the initial andfinal reflected intensity of the thin film layer as a function of radiusare shown in FIGS. 12 discussed above as well as in FIGS. 13A-13H.

Turning now to FIGS. 13A-13H, a number of traces which display howreflected intensity changes during polishing for different radialpositions on the substrate 10 are shown. The charts of FIGS. 13A-13Hillustrate that the metal layer is removed at different rates fordifferent portions of the substrate. Generally, FIGS. 13A-13H show thatthe metal layer near the center of the substrate is removed last, whilethe metal layer near the perimeter or edge of the substrate is clearedfirst. For example, FIG. 13A shows that the metal layer within a radiusrange of 0-20 mm is removed at about 330 seconds. FIG. 13B shows thatthe metal layer within a radius range of 20-30 mm is removed at about325 seconds. FIG. 13C shows that the metal layer within a radius rangeof 30-40 mm is removed at about 318 seconds. FIG. 13D shows that themetal layer within a radius range of 40-50 mm is removed at about 310seconds. FIG. 13E shows that the metal layer within a radius range of50-60 mm is removed at about 295 seconds. FIG. 13F shows that the metallayer within a radius range of 60-70 mm is removed at about 290 seconds.FIG. 13G shows that the metal layer within a radius range of 70-80 mm isremoved at about 290 seconds; and FIG. 13H shows that the metal layerwithin a radius range of 80-90 mm is removed as early as about 260seconds.

As shown therein, the reflectance trace for several of the radial rangesexhibit two intensity levels (shown by lines 160 and 162). The distancebetween the two intensity levels increases with substrate radius.Without being limited to any particular theory, the two intensity levelsmay be caused by non-symmetric distribution of the slurry or the productof the reaction of the slurry and the metal layer on the substrate.Specifically, on each sweep of the laser beam across the substrate, twodata points are usually entered in a bin: one data point which is closerto the leading edge of the substrate and one data point which is closerto the trailing edge of the substrate. However, due to non-symmetricdistribution of the slurry and the reaction products beneath thesubstrate, the laser beam may be more attenuated when passing throughslurry layer adjacent different regions of the substrate. Thus, thereflectance traces might also be used as a measure of the uniformity ofslurry distribution beneath the substrate.

In another implementation, an operator might decide to use only a singlebin. In this case, all of the intensity measurements for the specifiedradial range are used to determine a single intensity trace, which isused for determination of a polishing endpoint in the conventionalfashion. The operator can specify this radial range based on examinationof the transient signal graphs. For example, if the transient signalgraphs show that the center of the substrate is the last portion to bepolished, then the operator can select a radial range around thesubstrate center to ensure that the endpoint is not triggered until allof the metal has been polished away.

The reflection intensity changes during polishing are thus captured fordifferent radial positions on the substrate. The high resolution dataacquisition allows a precise time control of each process step in amulti-step operation. A wealth of parameters such as uniformity of theentire wafer and removal rate for different radial portions of the waferare captured. The acquired high resolution data can be processed on-lineor off-line to adjust various variables and parameters to minimizeerosion and dishing of the surface layer. If the data is processed inreal-time, the real-time feedback data allows a tighter closed-loopcontrol with the process parameters. Further, the reflection data isavailable for process engineers to experiment with their processingparameters to improve the polishing process.

The reflectometry traces generated by the optical monitoring system areparticularly useful in a multi-step polishing process, such as copperpolishing. Referring to FIG. 14, a substrate 10′ includes a siliconwafer 12′, a patterned oxide layer 14′, a tantalum (Ta) ortantalum-nitride (TaN) barrier layer 18 disposed over the oxide layer14′, and a copper layer 16′ disposed over the barrier layer 18.Referring to FIGS. 15A and 15B, as the 'substrate 10′ is polished, areflectance trace 200 is generated for each radial zone. Eachreflectance trace illustrates the bulk removal of the metal layer (flatregion 202), the initial clearing of the metal layer and transition tothe barrier layer (drop-off point 204), initial clearing of the barrierlayer and transition to the oxide layer (first decrease in slope point206), and complete clearing of the barrier layer and exposure of theoxide layer (second decrease in slope and flatting out of trace at point208).

Initially, the substrate is polished with a high-selectivity slurry,e.g., Cabot 5001. The bulk polishing of the metal layer 16′ proceedsuntil the initial break-through to the barrier layer 18. At this point,metal may still overlay portions of the barrier layer 18, but thebarrier layer will be exposed in at least one region. The opticalmonitoring system can detect this initial exposure of the barrier layer.Specifically, the first reflectance trace that begins to drop can beused as an indication that the barrier layer 18 has been exposed in theassociated radial zone. When the optical monitoring system detects theinitial exposure of the barrier layer, polishing with thehigh-selectivity slurry is halted, and polishing with a low selectivityslurry, e.g., Arch Cu10k, is initiated. Polishing with the lowselectivity slurry continues until all of the metal layer 16′ and thebarrier layer 18 have been removed. The complete removal of all of themetal layer 16′ and barrier layer 18′ may be indicated when all of thereflectance traces have leveled off. At this point, the polishingoperation is complete. By reliably switching from a high-selectivityslurry to a low-selectivity slurry at the initial clearance of thebarrier layer, dishing and erosion can be significantly reduced. Inaddition, by halting polishing only once all reflectance tracesindicate, complete removal of the barrier layer can be ensured.

The present invention has been described in terms of a preferredembodiment. The invention, however, is not limited to the embodimentdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A polishing system comprising: a polishingsurface; a carrier to hold a substrate against the polishing surface; amotor to move at least one of the polishing surface and the carrier headto generate relative motion between the polishing surface and substrate;a dispensing system to dispense a first polishing fluid and a secondpolishing fluid; an optical monitoring system; and a computer configuredto cause the polishing system to chemical mechanical polish a firstlayer of substrate with the first polishing fluid, optically monitor thesubstrate during polishing with the first polishing fluid to generateplurality of intensity traces, with each intensity trace includingintensity measurements from a different radial range on the substrate,once any of the intensity traces indicates an initial clearance of afirst layer on the substrate, chemical mechanical polish the substratewith a second polishing fluid having different polishing properties thanthe first polishing fluid, continue to optically monitor the substrateduring polishing with the second polishing fluid, and halt polishingafter all the intensity traces indicate that a second layer beneath thefirst layer has been completely exposed.
 2. The system of claim 1,wherein the optical monitoring system includes a light source thatdirects a light beam through a window in the polishing surface and adetector to receive reflections of the light beam off the substrate andgenerate a reflectance signal.
 3. The system of claim 2, wherein thedetector moves relative to the substrate to cause the light beam to movein a path across the substrate.
 4. The system of claim 3, wherein thecomputer is configured to extract a plurality of intensity measurementsfrom the reflectance signal.
 5. The system of claim 4, wherein thecomputer is configured to sort each intensity measurement of theplurality of intensity measurements into one of the radial rangesaccording to a position of the light beam during the intensitymeasurement, and to determine the intensity trace from the intensitymeasurements associated with the radial range.
 6. The system of claim 1,wherein the first polishing fluid is a high-selectivity slurry and thesecond polishing fluid is a low-selectivity slurry.
 7. The system ofclaim 1, wherein the computer is configured to detect a drop in anintensity trace as clearance of the first layer.
 8. The system of claim1, wherein the computer is configured to halt polishing when the secondlayer has been completely removed.
 9. The system of claim 8, wherein thecomputer is configured to detect a flattening out of the intensity traceas removal of the second layer.
 10. A polishing system comprising: apolishing surface having a window; a carrier to hold a substrate againstthe polishing surface; a motor to move at least one of the polishingsurface and the carrier head to generate relative motion between thepolishing surface and substrate; a dispensing system to dispense a firstpolishing fluid and a second polishing fluid, the second polishing fluidhaving different polishing properties than the first polishing fluid; anoptical monitoring system that includes a light source that directs alight beam through the window, wherein the motion of the polishingsurface relative to the substrate causing the light beam to move in apath across the substrate, and a detector that detects reflections ofthe light beam from the substrate and generates a reflectance signaltherefrom; and a computer configured to cause the polishing system topolish the substrate with the first polishing fluid in a first polishingstep, extract a plurality of intensity measurements from the reflectancesignal, generate a plurality of intensity traces, with each intensitytrace including intensity measurements from a different radial range onthe substrate, once any of the intensity traces indicates an initialclearance of the first layer, polishing the substrate with the secondfluid in a polishing second polishing step, and halt polishing after allthe intensity traces indicate that the second layer has been completelyexposed.
 11. The system of claim 10, wherein the first polishing fluidis a high-selectivity slurry and the second polishing fluid is alow-selectivity slurry.
 12. A polishing system comprising: a polishingsurface having a window; a carrier to hold a substrate against thepolishing surface; a motor to move at least one of the polishing surfaceand the carrier head to generate relative motion between the polishingsurface and substrate; a dispensing system to dispense ahigh-selectivity slurry and a low-selectivity slurry to the polishingsurface; an optical monitoring system that includes a light source thatdirects a light beam through the window, wherein the motion of thepolishing surface relative to the substrate causing the light beam tomove in a path across the substrate, and a detector that detectsreflections of the light beam from the substrate and generates areflectance signal therefrom; and a computer configured to cause thepolishing system to polish a metal layer on the substrate with thehigh-selectivity slurry, extract a plurality of intensity measurementsfrom the reflectance signal, determine a radial position for eachintensity measurement, divide the plurality of intensity measurementsinto a plurality of radial ranges according to the radial positions,generate a plurality of intensity traces, with each intensity traceincluding intensity measurements from one of the plurality of radialranges, supply the low-selectivity slurry to the polishing surface whenany of the intensity traces indicates an initial clearance of the metallayer, and halt polishing when all the intensity traces indicate thatthe oxide layer has been completely exposed.
 13. The system of claim 12,wherein the computer is configured to detect a sudden drop in areflectance trace as indicating initial clearance of the metal layer inthe radial range associated with the reflectance trace.
 14. The systemof claim 12, wherein the computer is configured to detect a flatteningout of a reflectance trace as indicating exposure of the oxide layer inthe radial range associated with the reflectance trace.